1. Technical Field
The present invention relates to the design of closure apparatus for high pressure vessels and, more particularly, to a closure providing both full and partial access.
2. Description of the Prior Art.
The design and operation of high pressure feedwater heaters in modern steam generating plants are subject to several constraints. Typical operating pressures on a high pressure side of such heaters in the range of 3,000-5,000 pounds per square inch (1,065-1,775 N/cm.sup.2) require thick walls, and, consequently, heavy structures. The need for access to inspect and replace feedwater tubes makes it desirable to have closures for high pressure vessels of such heaters that are relatively easy to remove and replace. Unfortunately, improved ease of access is usually accompanied by increased weight and complexity.
Conventional feedwater heaters are usually cylindrical shell and tube heat exchangers in which a bundle of many tubes is enclosed by a cylindrical shell. The tubes may either extend straight between tube sheets located at opposite ends of the shell, or they may be bent into a U-shape so that the inlet and outlet ends of each tube terminate in a single tube sheet. The interior of the tubes is usually the high pressure side of the heater; consequently, the shell wall can be relatively thin. The heavy construction can be limited to two end chambers (in the case of straight tube construction) or to a single end chamber (in the case of a U-tube construction). These end chambers, whether single or double, comprise high pressure vessels and are provided with inlet and outlet connections for the high pressure feedwater that flows through the tubes. The chambers, also called channels, are cylindrical, with an inner diameter large enough to provide full access to all the tube penetrations through the tube sheet. Because the wall thickness of the cylindrical high pressure end chamber is proportional to the diameter, for a given pressure, the internal diameter of the channel should be as small as is consistent with the need to gain access to the tubes. By way of example, a typical interior diameter of a feedwater heater channel is thirty-six inches, the thickness of the annular wall of the channel is eight and one-half inches, and the operating pressure is five thousand pounds per square inch.
In the petrochemical industry, high pressure vessels are employed in the refining and in the transmission of liquid petrochemical products. Such high pressure vessels may not be subject to the extreme thermal stress of high pressure steam generating plants. On the other hand, such high pressure vessels typically must withstand operating pressures in the range of 8000 pounds per square inch. Furthermore, it is equally desirable in the petrochemical industry to provide closures that are relatively easy to remove and replace.
There are two types of closures conventionally provided for such high pressure vessels, known as full access and partial access. The full access type is usually a circular flat disc or plate that fits slidably into the channel and is secured by sector pieces or blocks that fit into an inner circumferential groove in the channel wall. An example of this type of closure is shown in U.S. Pat. No. 4,463,871 of Garbarini et al., the disclosure of which is incorporated herein by reference.
The partial access type of closure is usually a hemispherical head that is butt-welded to the end of the channel wall and is provided with a concentric manway. The manway may have a bolted cover, or, if it has an elliptical cross section, it may have an internal closure which makes use of the internal pressure in the vessel to provide a leak-tight seal.
The partial access closure has the advantage that the relatively small manway cover can be removed quickly and easily to permit periodic inspection and preventive maintenance, for example, of the tubes in feedwater heater installations. If extensive repair or replacement of the tubes is needed, however, the hemispherical head has to be removed by cutting away the butt weld. After completion of the repairs, the head must be rewelded and the weld thoroughly inspected. This procedure is time-consuming and expensive.
The flat plate full access closure has the converse drawback that it must be removed even for a routine inspection. The flat plate closure is approximately twice as heavy as a hemispherical head of the same diameter, because the flat plate is subjected to principally bending stresses whereas the hemispherical head is loaded primarily in tension. Thus, for a 5,000 psi pressure chamber having a threefoot diameter channel opening, the closure plate may weigh six tons, and the individual sector pieces may be 200 pounds apiece. Removal and reassembly of the closure requires a large crew and special staging and equipment. Consequently, routine inspection and preventive maintenance involve considerably more expense than the hemispherical head, which has only a relatively small manway cover to handle. As a result, the period between inspections tends to be extended for feedwater heaters having a flat plate full access closure, giving more time for small problems to become large ones.
It would be desirable to have a closure that provides the advantages of both the full access and partial access types. The flat plate closure, however, does not permit enough space between the inner face of the plate and the opposed face of the tube sheet to allow full inspection even if a manway were provided through the cover. The spherical outer surface of the hemispherical closure, on the contrary, does not provide enough radial space to maneuver the sector pieces into position and to secure them in place with a wedge ring as used with the flat plate closure.